Please save me from this Grade 10 maths word problem. I am stuck!!

Karabo is training by running for the Comrades marathon, he starts his training by running 40km per week. Each week he increases the distance covered by 10km.

- Write down the formula to calculate the distance he runs in the n-th week. (4 marks)
- Use the formula to calculate the distance he runs in the 12th week of training (1 mark)